import java.util.Scanner;

public class Main {
    public static void main(String[] args) {

        Scanner s=new Scanner(System.in);
        int k=s.nextInt();
        String str1=s.next();
        int count=0;
        char ch=str1.charAt(0);
        for (int i = 0; i < str1.length(); i++) {
            if(str1.charAt(i)==ch){
                count++;
                if(count==k){
                    System.out.println(ch);
                    System.exit(0);
                }
            }else{
                ch=str1.charAt(i);
                count=1;
            }
        }
        System.out.print("No");
    }
}